Instrumented Micropiles Micropiles Instrumented Robert Taylor, P.Eng. International Workshop on Micropiles Schrobenhausen, May 2006
Micropiles Micropiles • Titan Bars Titan Bars • • Threaded Threaded • • Hollow Hollow • • Drill and install in one operation Drill and install in one operation • • Through Through- -grouted grouted • • Sizes 30/16 to 73/53 OD/ID Sizes 30/16 to 73/53 OD/ID • • UTS 220 UTS 220- -8000 8000 kN kN •
Micropile Strain Gauge Strain Gauge Micropile 150mm VW strain gauges � 150mm VW strain gauges � Installed after drilling – – no damage no damage � Installed after drilling � Uses grout that’s already there � Uses grout that’s already there � Multiple installations possible � Multiple installations possible � Protected inside micropile micropile � Protected inside � Reliable multi- -zone load monitoring zone load monitoring � Reliable multi �
Sensor Design Sensor Design � Vibrating wire embedment gauge Vibrating wire embedment gauge � � Excellent immunity to cable moisture Excellent immunity to cable moisture � � Cable readily shortened/extended without Cable readily shortened/extended without � effect on results effect on results � 150 mm length gives averaging of grout 150 mm length gives averaging of grout � discontinuities discontinuities � Compliant carrier centers the gauge on the Compliant carrier centers the gauge on the � neutral axis, facilitates passage through neutral axis, facilitates passage through couplings couplings
Installation Method Installation Method � Drill and fully grout pile Drill and fully grout pile � � Finish with high Finish with high- -strength cement grout strength cement grout � – e.g. W/C 0.45 e.g. W/C 0.45 – � While grout still fluid, push in VW strain While grout still fluid, push in VW strain � gauge using low- -modulus rods modulus rods gauge using low � Readings as soon as grout sets Readings as soon as grout sets � � Readings during load application Readings during load application �
Test Program Test Program � To facilitate interpretation of data from To facilitate interpretation of data from � micropile strain data, controlled load of strain data, controlled load of micropile instrumented bar made instrumented bar made � Because Because micropiles micropiles can be applied in tensile can be applied in tensile � and compressive directions, both load and compressive directions, both load directions tested directions tested � Tensile test made with hollow jack Tensile test made with hollow jack � � Compressive test made on shortened bar Compressive test made on shortened bar �
Specimen Preparation Specimen Preparation � On production site, test 73 x 53 OD x ID On production site, test 73 x 53 OD x ID � pile drilled 40 cm into soil, grouted, and pile drilled 40 cm into soil, grouted, and gauge inserted gauge inserted � Following day grout found to have escaped, Following day grout found to have escaped, � exposing gauge exposing gauge � Bar Bar regrouted regrouted, tapped to release bubbles , tapped to release bubbles � � Grout cured 25 days in soils laboratory Grout cured 25 days in soils laboratory �
Tensile Test Test Bar Hydraulic Load Jack Cell
Compressive Test
6000 5000 Titan Bar 73/53 with 6" VWSG-E Strain Gauge cf RST VWA-220-3.937 S/N VC1300 4000 microstrain 3000 Initial Loading Cycle 2000 1000 Predicted strain from COSMO FEM model 0 -1000 -800 -600 -400 -200 0 200 400 600 800 1000 Load, kN (tension+)
Results Results � Proportional response Proportional response � � Minor non Minor non- -linearity on first loading to upper linearity on first loading to upper � values values � Recovery and reloading response linear, Recovery and reloading response linear, � repeatable; matches prediction of finite repeatable; matches prediction of finite element model element model
Inspection Inspection � After testing, the specimen bar was After testing, the specimen bar was � sectioned longitudinally sectioned longitudinally � The sectioning cuts were made by end The sectioning cuts were made by end � milling, flooded in coolant to a residual milling, flooded in coolant to a residual thickness of 1.25 mm thickness of 1.25 mm � Coolant was excluded using plastic mastic Coolant was excluded using plastic mastic � � The last 1.25 mm was cut using high The last 1.25 mm was cut using high- -speed speed � dry abrasive wheel dry abrasive wheel
Inspection continued ontinued Inspection c When first side metal was cut through, the When first side metal was cut through, the � � micropile sprung open, with the grout sprung open, with the grout micropile remaining tightly retained on one side remaining tightly retained on one side � The grout had to be pried out, resulting in The grout had to be pried out, resulting in � transverse fracturing in what appeared to be transverse fracturing in what appeared to be intact material intact material � Artifacts of specimen preparation problems Artifacts of specimen preparation problems � (bubbles, parting at regrouting regrouting surface) had surface) had (bubbles, parting at no apparent effect on performance no apparent effect on performance
Recommendations Recommendations � It is desirable to install into a fully grouted It is desirable to install into a fully grouted � pile, and to avoid subsequent loss of grout – – pile, and to avoid subsequent loss of grout a positive grout retention system should be a positive grout retention system should be used used � Caution should be used in inferring load Caution should be used in inferring load � from strains on initial loading from strains on initial loading µε ), peak strains (>2000 µε � For large peak For large peak- -peak strains (>2000 ), � biasing installation strains and/or biasing installation strains and/or desensitized gauges required desensitized gauges required
Conclusions Conclusions � After initial loading, subsequent unloading After initial loading, subsequent unloading � and reloading are typically very linear and reloading are typically very linear � The method appears a robust, practical way The method appears a robust, practical way � to measure load in injection bored to measure load in injection bored micropiles micropiles
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